The present invention relates to an improvement of a grid for use in a lead storage battery.
Today there is a great desirability for a reduction in weight of lead storage batteries in order to improve the fuel economy in automobiles fitted with a storage battery, to increase the running distance performance of electric powered vehicles, and to improve portability of portable devices.
In typical lead storage batteries, the grid is produced by casting lead alloy. In spite of efforts to reduce the weight of the grid of the material having a large specific gravity of, for example, 11.3 for pure lead by making the grid thin, technical difficulties are still encountered in the production of a grid of 1 mm or less thickness by casting. In addition, the thin grid tends to be deformed in the subsequent paste filling process or curved during handling. For these reasons, the reduction in weight of the cast grid is limited.
In the past, therefore, it has been devised to partly exchange the lead alloy with synthetic resin having about 1/10 of the specific gravity of lead alloy to reach various types of a so-called composite grid of lead alloy and synthetic resin wherein lead alloy is used for only the portion which is required to be conductive and synthetic resin is used for the portion which is exclusively adapted to support active materials.
For example, in a lead alloy/synthetic resin composite grid disclosed in (A)Japanese Patent Kokai (Laid-open) No. 8037/75, the portion of grid required to be conductive is first cast with lead alloy, and the cast portion is then inserted in a matrix for synthetic resin moulding and synthetic resin is poured in the matrix to form, by moulding with synthetic resin, the portion for support of the active materials which unite with the lead alloy portion.
Disclosed in (B) Japanese Utility Model Publication No. 3535/68 is another lead alloy/synthetic resin composite grid wherein a grid of synthetic resin and a perforated plate of lead alloy are laminated and bonded together.
The production process for each of the above composite grids is, however, complicated and in addition, the composite grid having the conductive portion of lead alloy in the form of slender ribs or a thin plate tends to be curved or deformed and is therefore difficult to handle. Accordingly, these composite grids are successful in reducing weight to a certain extent but they suffer from poor producibility and are unsuited for mass production.
Further, a composite grid produced by a metal spraying process has also been proposed wherein fine particles of molten lead are sprayed at high speed onto the surface of a grid of synthetic resin to penetrate the synthetic resin grid surface and be held therein. This composite grid can advantageously be produced with simplified equipment but on the other hand, the utilization efficiency of the sprayed molten lead is poor in the metal spraying process. And, the film thickness becomes irregular and especially, the film has a small-thickness portion in which intimacy with the synthetic resin grid is poor so that the lead particles remain as they are, resulting in a porous coating. Accordingly, the lead particles per se covering the synthetic resin grid are easily reacted with and are liable to be turned into an active material and hence, from the standpoint of the function of collecting current from the paste active material, electrical resistance is increased and the composite grid used as a plate of storage battery is degraded in voltage characteristics and life.